Source apportionment and ecotoxicity of PM _2.5 pollution events in a major Southern Hemisphere megacity: influence of a biofuel-impacted fleet and biomass burning

Abstract. The Metropolitan Area of São Paulo (MASP) in Brazil has reduced its vehicular emissions in the last decades. However, it is still affected by air pollution events, mainly in the winter, characterized as a dry season. The chemical composition of fine particulate matter (PM2.5) was studied in the MASP during a 100 d dry period in 2019. PM2.5 samples underwent an extensive chemical characterization (including inorganic and organic species), ecotoxicity was assessed using a bioluminescence-based assay, and submicrometer particle number size distributions were simultaneously monitored. PM2.5 concentrations exceeded the new World Health Organization's daily guidelines on 75 %​​​​​​​ of sampling days, emphasizing the need for strengthening local regulations. Source apportionment (positive matrix factorization, PMF5.0) was performed, and the sources related to vehicular emissions remain relevant (over 40 % of PM2.5). A high contribution of biomass burning was observed, reaching 25 % of PM2.5 mass and correlated with sample ecotoxicity. This input was associated with north and northwest winds, suggesting other emerging sources besides sugarcane burning (forest fires and sugarcane bagasse power plants). A mixed factor of vehicular emissions and road dust resuspension increased throughout the campaign was related to stronger winds, suggesting a significant resuspension. The sulfate secondary formation was related to humid conditions. Additionally, monitoring size particle distribution allowed the observation of particle growth on days impacted by secondary formation. The results pointed out that control measures of high-PM2.5 events should include the control of emerging biomass-burning sources in addition to stricter rules concerning vehicular emissions.